Transparency turns black after Math:Multiply node. Need help
The problem is that the math node uses a value (=> grey dot) and not a color (=> yellow dot). You'd rather use a RGB Curves node (ShiftA => Color => RGB Curves) and change it's C value. This will give you what you want.

 
Look at how the backdrop changes. The first time it's the normal render without any compositing. It still has an alpha channel. The second time it's with the math note. The alpha channel disappears. The third time it's with the RGB Curves node. The colors get strengthen but the alpha channel stays.